Electromagnetic phonograph pickups



July 10, 1962 J. H. MCCONNELL ELECTOMAGNETIC PHONOGRAFH PICKUPS 2 Sheets-Sheet 1 Filed Feb. 15, 1957 INVENTOR.

BY John H.McConnel| ATTORN EYS United States Patent 3,043,920 ,ELECTROMAGNETIC PHQNOGRAPH PICKUPS John H. McConneli, MurrayI-lill, N..I., assignor to Electro-Sonic Laboratories, Inc, Long Island City, N.Y., a corporation of New York Filed Feb. 15, 1957, Ser. No. 640,427 7 Claims. (Ci. 179-10041) This invention relates to phonograph pickups, and more particularly to electromagnetic phonograph pickups wherein the oscillation of the stylus causes an electromagnetic transducer to generate corresponding electric signals in windings interlinked with the magnetic core of the transducer structure. In the best prior-art pickups of the foregoing type, a narrow pin-shaped magnetic armature carrying elongated windings was oscillatably mounted between pole pieces of a permanently magnetized core structure so that when the pin-like armature is oscillated about its axis by the groove-engaging stylus, the magnetic flux of the core structure will induce corresponding electric signal output in the core windings of the armature.

Among the I objects of the invention is a simplified phonograph pickup of the foregoing type, wherein the pin-like armature does not have to carry any winding, and has otherwise all desired characteristics of such best prior pickups while avoiding the difficulties and limitations inherent therein. Among the objects of the invention is also an electromagnetic pickup wherein the magnetic core elements and windings constitute a shielded system which suppresses interlinkage of disturbing magnetic fields with the signal-generating elements of the pickup.

The foregoing and other objects of the invention will be best understood from the following description of an exemplification of the invention, reference being had to the accompanying drawings, wherein:

FIG. 1 is a vertical cross-sectional view along lines 11 of FIG. 2, of one form of pickup exemplifying the invention; 7

FIG. 2 is a top view of the pickup of FIG. 1, with a part broken away to show some details thereof;

FIG. 3 is a front view of the pickup of FIG. 1 as seen from the left side of FIG. 1;

FIG. 4 is a cross-sectional view of the front part of the pickup along lines 4-4 of FIG. 1; v

FIG. 5 is an elevational view of the front part of the core structure of the pickup of FIG. 1;

FIG. 6 is a cross-sectional view along lines 6-6 of FIG. 5;

FIG. 7 is an elevational view of the assembled armature mounting unit as seen from the left or front side of FIG; 1;

FIG. 8 is a cross-sectional view along lines 8-8 of FIG. 7; a

FIG. 9 is a cross-sectional view along lines 9-9 of FIG. 7; and

FIG. 10 is a bottom view along lines 10-10 of FIG. 7, of the armature mounting unit assembly.

FIGS. 1 to 10 show one form of a pickup of the invention designed for reproducing records from laterally cut record grooves although a similar pickup structure may also be used for reproducing vertically cut record grooves. The pickup shown comprises a main magnetic core structure 10 of low magnetic retentitivity which is interlinked with core windings 11 and constitutes a substantially closed magnetic circuit with pole faces 12, 13 facing each other across a relatively narrow pole space 14 separating the same. In the form shown, the main magnetic core structure 10 consists of a central, inner core arm or pole 15 separated by a surrounding coil space 16 from an outer core arm or pole 17 which surrounds on all sides the inner core arm 15 and the coil space 16. The free end "ice of the inner core arm 15 has the pole face 12 facing the opposite pole face 13 of the core 10. The other end of the inner' core arm is joined through a continuous body 18 of the same magnetic material to the outer core arm 17. The magnetic core path of the outer core arm 17 is completed by a complementary magnetic bridge or wall member 21 which forms a continuation of the outer core arm 17 of core structure 10, and has the complementary pole face 13 separated by pole space 14 from the other pole face 12. The core bridge member 21 is suitably secured, as by cementing, to the outer core arm 17 of the core structure 10 along their respective faces 17-1 (FIGS. 1, 2 and 4). For all practical purposes the magnetic body of the main core structure 10 with its bridge wall member 21 form a continuous magnetic casing of low magnetic retentivity enclosing on substantially all sides the coil space 16 and also the pole space 14, separating its pole faces 12, 13, so as to suppress Penetration of external disturbing leakage fluxes through the pole space 14, the coil space 16 and through the windings 11 interlinked with the core 10. 1

In the narrow pole space 14 separating the pole faces of the core structure 10, is mounted a permanently magnetized magnetic rotor member 30 which is arranged for rotary oscillatory movement along its central elongated axis 31 extending through the pole space 14. It is assumed that the pickup shown will reproduce groove records of a disc extending in a horizontal plane, in which case the motor axis will extend in vertical direction. The magnetic rotor 30 is permanently magnetized so that when it is in neutral condition shown, its axis or direction of permanent magnetization, indicated in FIG. 4 by arrow 32, extends generally parallel to the pole faces 12, 13 of the core 10, in a direction substantially perpendicular to its rotor axis 31. When the magnetic rotor 30 is stationary in its neutral position shown, the magnetic forces of its permanent magnetization will cause it to induce a magnetic flux which will be balanced relatively to the core structure 10 and its windings, and in such stationary neutral position no voltage will be induced in the core windings 11 of the core structure. However, any minute oscillatory movement of the permanently magnetized rotor 30 in either direction around its oscillation axis 31, will induce varying alternating magnetic flux through the core parts which are interlinked with the windings 11, and will generate therein a voltage corresponding to the oscillatory movement of the rotor 30. The magnetic rotor 30 thus forms with the casing core 10 and the core windings 11 a magnetic bridge system which automatically shields the core windings 11 against interlinkage with disturbing external magnetic flux fields, and thus Suppresses pickup of undesired noise signals.

For superiorresult-s, the magnetic rotor 30 should consist of permanently magnetizable sintered' metallic oxide material known as ferrite of the type described in the article. Magnetic Ferrites, published in Electrical Manufacturing, December 1949, by Snyder, Albers- Schoenberg and Goldsmith, and in the publications referred to therein. In order to reduce the moment of ine'rtia of the. magnetic rotor to a minimum, its cross-sectional area or width is kept at a practical minimum, such as 0.020 inch to 0.050 inch or up to about 0.060. Since sol-id ferrite material is made of sintered ferrite particles and is relatively brittle, a thin magnetic rotor of such ferrite material as available at present would have only limited strength and would require careful handling. According to the invention, these difficulties are avoided by making the principal magnetic rotor part of the pickup relatively short and by combining therewith a strong supporting rod or pin on which the magnetic ferrite rotor is held fixed in its proper operative position between the pole faces 12, 13 of the magnetic core 10. In the form pin 35 of permanently magnetizable steel and passing it through a central hole of the magnetic'i'errite rotor 30 which is suitably secured in its operative position on the rotor pin 35, as by cement.

The downward end of the rotor 30 which is outside the pole space 14, as seen in FIG. 1, is provided with a laterally extending drive or stylus arm 36. A stylus 37 shaped for engagement with a record groove such as a laterally cut record groove, has its axis laterally displaced from the oscillation axis 31 of the rotor 30, and is arranged to drive or oscillate the stylus arm 36 either by a direct connection thereto or by a detachable coupling thereto, for instance in the manner described in Klingener Patent In the form shown, the stylus arm 36 has a mounting collar which is secured as by riveting to the lower end of the rotor pin 35. Means are also provided for pivotally supporting the rotor 30 for oscillation around its axis 31. In the form shown, the thin rotor pin 35 is rotatably held in two suitably mounted spaced hearing supports or bearings '38 foroscillation around the oscillation axis 31 of the rotor 30. The stylus arm 36 is very short and it has high rigidity or stiffness for lateral oscillatory forces transmitted to the rotor 30 for oscillating itabout its oscillation axis 31. .I-Iowevcr, the stylus arm 36 has low stiffness or great compliance for forces parallel to the rotor axis 31 to prevent transfer of vertical stylus forces to the rotor, thus eliminating or suppressing the pinching effect.

In accordance with the invention, the magnetic rotor 30 is combined with a distinct mounting member into a rotor assembly which carries all the elements of the permanently magnetized rotor 30 including its stylus arm 7' 36 and its bearings 38 in their operative positions, and

which permits ready withdrawal of the rotor assembly from or its ready replacement in its operative position within the core structure 10.

One form of rotor assembly for a pickup of the invention is shown in FIGS. 7-10, this assembly also being seen in FIGS. l-4 in its operative position within the pickup. The rotor assembly (FIGS. 7-10) comprises a frame or mounting member 40 having in its lower section 41 as seen in FIGS. 7 and 8, the two bearing supports 38 which support the rotor 30 in its operative oscillating position.

As s'hownin FIG. 9, one or both bearings 38 may be formed of rubber or like elastomer material with a bearing opening within which the cylindrical portion of the rotor pin 35 is seated and guided when it is rotated by the stylus drive arm 36. One of the elastomer bearings 33,

namely the lower one whichis adjacent the stylus arm 36, is mounted so as to frictionally engage the portion of the rotor 30 or rotor pin 35 seated therein for exerting thereon elastic restoring forces which automatically return the rotor 30 to its neutral position and linearly resist oscillation of the rotor 30 away from its neutral position. The other bearing 38 serves merely to provide for additio-n'al rotary support for the rotor 30 Without exerting thereon any other restraining forces. In practice, good results are obtained by making both bearings 38 of elastomcr material which is sufliciently compressed for exerting on the rotor 30 the desired amountof elastic restoring forces sutficient to return the rotor to its neutral position are forced or moved into their proper operative position 'Withintwo bearing slits 38-1 formed iuthe lower section 41 "ofithe rotor mounting member 40. The bearing slits 38-1 of the mounting member 40 are of somewhat smaller width than the combined thickness of the rotor pin section 35-1, and the eleastomer bearing strips 38 are folded thereover so that in the assembled position the surrounding el-astomer strip portions 38 are held slightly compressed within the bearing slits 38-1 of the mounting member 40 for assuring slight compression of the elastomer bearings 38 which will exert the minimum amount of elastic restoring forces on the rotor pin 35 seated therein,- as it is being oscillated relatively to its neutral position by the stylus arm 36 in accordance with the undulations of the record groove.

The rotor mounting member 40 is provided with elongated exterior aligning and guide surfaces 43, 44, 45, shaped for aligning engagement with corresponding aligning surfaces of a rotor aligning compartment of the core structure extending along its pole space 14, and which will herein be designated as pole-space compartments 14. The rotor aligning compartment 14 of the core structure 10 has a rectangular opening 51 (FIG. 1) on the downwardly facing side of the core structure 10, through which the upwardly extending rectangular main part of the mounting member 40 is inserted into its operative position within the pole space compartment 14 of the core, as seen in FIGS. 1, 3 and 4. The downward section of the mounting member 40, as seen in FIGS. 1, 7 and 8, has a laterally projecting shoulder portion 52 with an upper aligning surface 53 which fits against a downwardly facing aligning surface 54 of a recess formed in the downwardly facing front wall 21 of the core structure 10 for receiving therein and holding aligned in operative position the rotor mounting member 40 with its aligning shoulder portion 52. The shoulder portion 52 of the rotor mounting member 40 has two downwardly projecting stylus guards 56 which serve to limit the lateral excursions of the stylus arm36, The frame-like rotor mounting member 40 has on the downward face of its upper cross piece 47 overlying the upper end of rotor pin 35, a thrust bearing member 48 against which the rounded upper end of the rotor pin 35 is seated. The bearing member may be a jewel bearing of the type used in watches, or a flat sheet of hard material such as hard metal, or of a layer of rubber or elastomer material, such as used for the other bearings 38 of the rotor pin 35. The rotor mounting member 40 is made of suitable non-magnetic material. In practice, good results are obtained by'molding it of the known solid resin materials, as by injection molding.

In accordance with the invention, the magnetic rotor 30 is formed of one or more disc elements 30-1 of sintered permanently magnetizable ferrite material, each having a central openingshaped to fit over a cylindrical rotor pin 35 on which they are held. Such minute ferrite diso: 30-1 are available on the market for use as permanently magnetizable elements for computers. For purposes of the invention, the rotor discs need not have the rectangular type of hysteresis loop which they require for computer applications, as similar ferrite rings having any type of hysteresis loop which gives the ferrite ring strong permanent magnetic characteristics are sufficient for. inducing by rotation around its axis 31, an alternating varying .flux in the magnetic core structure interlinked therewith. Such commercially available computer ferrite rings have a diameter of about .050 inch' and a height of about .025 inch, with a central opening of about .015 inch, with which it is seated on the metallic rotor pin 35. A pickup of the type shown, having a rotor with only one such standard computer-type ferrite disc, delivers an open-circuit voltage of about 3 millivolts at stylus velocity of 7.5 centimeters per second. A pickup of thetype shown having a rotor 30 with only three such standard computer-type 70 ferrite discs delivers an open-circuit voltage of about millivolts at such' stylus velocity. Accordingly, a simple pickup of the invention delivers a signal output of the same level as that obtained with widely used electromagnetic pickups of the reluctance type, such as described 75in Bachrnan Patent No. 2,511,663, the Pickup. of the in:

vention operating with at least the same degree of high fidelity and low stylus pressure. Where desired, the magnetic rotor 30 may be made of greater axial length or with more ferrite rings.

In accordance with the invention, the closed main magnetic core including its magnetic wall member 21 is formed of low-retentivity, high-permeability sintered ferrite material of the type which is widely used in the radio and television industries for making cup-shaped magnetic transformer cores and other magnetic core elements for electronic circuits. Such cup-shaped, lowretentivity pickup of the invention is made of such commercially available magnetic ferrite cores without requiring special tooling for producing with them the pickups of the invention on a commercial basis. Such commercially available low-retentivity ferrite cores have a cylindrical shape %ths inch outer diameter and inch high. The inner core arm is of cylindrical shape and is inch in diameter. The radial thickness of the outer core body 17 is 1 inch, this also being the thickness of the core portion 18 joining the inner core arm 15 to the outer core arm 17. The complementary core wall 21 which completes the core 10 is likewise made of a standard commercial cup-shaped core of the type shown, with the same general dimensions, but of shorter axial height, and from which the central core corresponding to the core 15 has been removed to provide for the pole space 14 between the pole face 12 of the inner core 15 and the opposite pole face 13 formed by the central facing surface portion of core wall 21.

A pickup of the invention having the same output and performance characteristics as described above, may be made with a much smaller magnetic core structure 10, forming with such magnetic rotor the same balanced magnetic core structure which automatically shields all interior operating circuits against disturbing stray fluxes. The pickup of the invention may also be given a different shape, for instance, a narrower rectangular shape which will fit within a narrow, generally rectangular space of the forward end of the tone arms of the type used in conventional phonographs.

As explained above, the surfaces ofthe core structure 10 and its core wall 21, which face the pole space compartment 14, are provided with aligning and guide surfaces which assure proper guidance and alignment of the rotor mounting member with its rotor 30 when slidably returned into the pole space compartment 14 along its aligning and guide surfaces 43, 44 and 45. In order to make it possible to use for the core structure 10 and its complementary core wall 21, commercially available transformer core elements of the type shown, the windings 11 of the core structure 10 are provided with a bobbin structure 61 shaped to provide guide walls with guide surfaces along which the guide walls 45 of the rotor mounting member 40 are guided into their operative positions within the pole space compartment 14 of the core structure 10. The bobbin structure 61 is of conventional type and made, for instance, of strong synthetic resin material, by conventional molding procedures. The bobbin 61 has a cylindrical core and two transverse end Walls 63, 64 defining between them the coil space Within which the coil windings 11 are confined. The transverse bobbinend-wall 64 positioned adjacent the pole space 14, is provided with two wall projections 65 extending lengthwise of the pole space 14 and engaging with their facing guide surfaces the elongated guide surfaces 45 of the rotor mounting structure for guiding it and holding it in its aligned position between the opposite pole faces 12, .13 of the core structure 10, which form the other guide surfaces for the rotor mounting structure 40. The guide wall projections 65 of the bobbin 61 extend within the pole space compartment 14 for at least the same height as the upper frame-like part of the rotor mounting member 40 projects above its wider lower shoulder section 52. This arrangement assures that the guide surfaces 43, 44, 45 of rotor mounting member 40 are positively guided by the guide surfaces provided by the facing pole faces 12, 13 of the core structure 10, and by the facing guide surfaces of the bobbin projections 65, into the proper operative position of the magnetic rotor 30 within the pole space compartment 14 when it is replaced therein. The frictional engagement between the guide surfaces of the rotor mounting member 40 with the complementary guide and pole faces 12, 13 of the core structure and the facing guide surfaces of the coil bobbin 61, is sufiicient for retaining the rotor mounting member 40 with its rotor 30 in its proper operative position within the core structure 10 while permitting ready sliding removal of the rotor assembly from the core structure 10.

The distance between the two pole faces 12, 13 is only slightly greaterthan the diameter of the magnetic rotor 36, of the order of about .001 to .010 inch greater, just sufficient to suppress friction between the magnetic rotor 30 and the pole faces 12, 13.

The pickup shown is also provided with means for securing it, as with screws, in its operative position, such as in the tone arm of a phonograph (not shown). In the form shown, a rectangular mounting block 71, for instance of molded strong synthetic resin material, is provided with a slightly recessed wall surface 72 shaped to fit against an end surface of the facing core structure 10 to which it is suitably secured, as by a thin film of cement. The rectangular mounting block 71 is provided with two mounting holes shaped for receiving therein mounting screws '74 with which the pickup is secured to a mounting portion of the overlying part of the phonograph tone arm. Within the mounting block 71 are embedded two metallic terminal members 75, such as Wires, the inner ends of which project into a recessed compartment 76 facing the rear wall of the core. End leads of winding coil 11 of the core 16 pass through core perforations into mounting block compartment 76 and they are connected as by soldering to the terminal wires 75 before the mounting block '71 is secured to the core 10.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with a specific exemplification thereof, will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific exemplification of the invention described herein.

I claim:

1. In a phonograph pickup: a permanently magnetized rotor core held for oscillatory rotation about a rotor axis and permanently magnetized along a magnetic axis trans- ;verse to said rotor axis between two oppositely-directed,

opposite-polarity rotor pole faces; a hollow magnetic core enclosure of low magnetic retentivity enclosing with its interior core surfaces all sides of said rotor core; said core enclosure having along its interior core surfaces at least one inward pole projection with a pole end face and an opposite pole face on the opposite portion of said interior core surfaces, windings surrounding said pole projection, the two opposite core pole faces along said interior core surfaces being magnetically interlinked with said two rotor pole faces and passing flux induced by said rotor core through said windings and through two distinct opposite flux paths of said core for generating in said windings a signal output responsive to oscillatory motion of said rotor, said hollow core enclosure constituting a closed magnetic core shield enclosing on all sides the entire axial length of said rotor core and all non-magnetic gaps separating the two rotor pole faces from said core pole faces and suppressing disturbing inter-linkage of external flux with said windings.

2. In a phonograph pickup as claimed in claim 1, said core enclosure being formed of two complementary core sections, one of said complementary core sections being hollow and having a central pole projection extending through its interior hollow space toward an open wall side thereof; coil windings surrounding said core projection and confined within a hollow interior space extending between said pole projection and the surrounding wall portions of said one core section; the other of said complementary core sections fitting against the open wall side of said one core section and enclosing with its core walls a hollow rotor compartment extending in front of said pole projection and confining said rotor core.

3. In a phonograph pickup as claimed in claim 2, a selfesupporting, unitary rotor mounting structure of nonmagnetic material held in and insertable into said rotor compartment through a wall opening of said other core section located in a region spaced from the two pole faces of said core enclosure; and holding means for holding said rotor core in an oscillatory position on said rotor mounting structure.

4. In a phonograph pickup as claimed in claim 3, interiorly-facing surface portions of said two core sections and'exterior surfaces of said mounting structure constituting complementary interfitting, parallel guide surfaces for retaining and guiding said mounting structure as it is held and moved through said opening into its operative position within said rotor compartment.

5. In a phonograph pickup as claimed in claim 3, interiorly-facing surface portions of said two core sections and said mounting structure constituting interfitting guide surfaces extending in a direction generally parallel to said rotor axis for retaining and guiding said rotor mounting structure as it is held and moved through said opening into its operative position within said rotor compartment.

6. In a phonograph pickup as claimed in claim 5, said mounting structure having an interior hollow frame space,

said rotor core being held by said holding means within said frame space of said mounting structure.

7. In a' phonographpickup: an elongated, oscillatory magnetic core structure; a hollow magnetic core enclosure structure of low magnetic retentivity enclosing with its interior core surfaces all sides of said oscillatory core structure; said core enclosure structure having along its interior core surfaces at least one inward pole projection with a pole end face and an opposite pole. face on the opposite portion of said interior core surfaces; means for holding said oscillatory core structure for oscillatory movement between the opposite pole faces of said core enclosing structure, said core structures including a permanently magnetized core element for inducing a unidirectional magnetic flux through a core path including said pole faces and said two core structures, and windings interlinked with said core path for generating a signal output responsive to oscillatorymotion of said oscillatory core structure between said pole faces, said hollow core enclosure structure constituting a closed magnetic core shield enclosing on all sides the entire axial length of said oscillatory core structure and all non-magnetic gaps separating said oscillatory core structure from said core pole faces and suppressing disturbing interlinkage of external flux with said windings, said core enclosure structure having a relatively small wall opening aligned with said oscillatory core structure for enabling removal of said oscillatory core structure from between said pole faces by longitudinal movement generally transverse to its said oscillatory movement.

References Cited in the file of this patent UNITED STATES PATENTS 2,101,665 Arey et al. Dec. 7, 1937 2,501,233 OBrien Mar. 21, 1950 2,591,996 Arentzen Apr. 8, 1952 2,883,478 McConnell Apr. 21, 1959 FOREIGN PATENTS 896,121 Germany Nov. 9, 1953 OTHER REFERENCES N. Wittenberg: Philips Technical Review, vol. 18, 101- 144, 1956/57. 

